WO-A-2007/077389 discloses Hall effect sensors regularly distributed around a magnetic ring to supply sinusoidal type electric signals that enable the angular position of a rotary element to be determined by computation. If one of the sensors is faulty, then the complete system becomes non-operational.
US-A-2006/0055259 discloses a fault-tolerant magnetic bearing where several sensor sets are used to sense rotor displacements in transverse directions. If a displacement sensor does not work correctly, its output signal is disregarded and compensated, which decreases the accuracy of the measure.